Choice of blade: Glass fibre or Carbon fibre?

[ptoot]

I have been told that the main difference (beside weight) between Glass Fibre and Carbon Fibre blades is the way they bend.

Apparently a carbon blade can bend both longitudinally and transversely; what are the advantages of that (in comparison with a glass fiber blade).

Thanks a lot!

 

[leven]

its not the material itself, the material of course matters but its more a combination of the material (a fibre), epoxy, and the direction of the fibre..
Along the fibre it becomes more stiff than any other direction..

So if the weave is crossed like so: # it is stongest in the up>down direction and side to side, not diagonally ( / and \ ).

 

[ptoot]

@leven: what would be the advantage of the blade bending sideways (having the # pattern of fibres)?

thanks!

 

[leven]

no advantage at all, you want the bending to be harder so it flexes back when you bend it.
With # the fin would be stiffer at the angles you dont want the fin to bend in..

It is less stiff at diagonal angles, not desirable but not as serious, can cause some v-bending when used..

ok, art time, acsii monofin

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If you place the fibers like that the fin will be stiffer in the bending when you kick, and the bending of the "wings" (the four # at the ends).
But on the diagonal the fibers will not be as strong, and will force the epoxy to do more job., it will be less "spingy".

The discussion about the two materials is that Glassfiber is said to return more energy than Carbonfiber after bending, but the only ones i have heard saying that was monofin makers who used glassfibre, so i could mean that they have tried or that glassfiber is better or that it doesnt really matter and its expensive to produce in carbon..

There is a difference in the materials of course, carbon is much more expensive and harder, and lighter, glassfibre is a bit more flexible and cheaper, and heavier.
But how it performs is upp to how you use the materials, how many layers of fibre, what kind of expoxy, what kind of blend in the epoxy, how the layers are placed (you can place one layer diagonally and the next straight, creating stiffness in both angles but more thickness)..

 

[ptoot]

thanks for the effort you have put into this answer, that is one cool ASCII art fin!

 

[Old Man Dave]

Good answers but unless you are really into the physics of it all then you can probably just learn what you want from "general" knowledge.

Basically carbon fins are the sports cars of the fin world. Powerful, expensive and easily damaged. Fibreglass are the hot hatchbacks with good performance, just affordable prices but tough enough to survive daily use. Thermo plastic blades are Fords .

Dave.

 

[monkeyhatfork]

I think carbon is highly over-rated, if you look at the best mono they are glassfiber, I have a pair of c4 carbon bi-fins and while they feel very nice they don't perform any better than my plastic fins so slightly dissapointed that I spent so much money for them plus I have to wory about them more when I travel.

 

[Zmagoj]

I am of the opinion that /\ direction of the fibre results in stronger force needed to bend the fin. The reason is that fibres of both directions contribute to that stiffness. In mathematical terms (theoretically) that kind of orientation results in about 40% higher stiffness in the blade bend direction.

2*cos(45°)=1,41 -> 41% stiffer

I also started a thread in bifin section about self-made fiberglass fins, where I would like to discuss these aspects.

Regards,
Zmago

 

[kkdavids]

in my opinion through playing around with many fins, the difference between carbon and fibre is minimal..Carbon is supposed to be stronger (which i reality doesnt make them last any longer)....weight wise carbon are lighter, but this doesnt always increase performance, as having the extra weight can be beneficial depending on your fin stroke...and to be a complete hypocryte i main fins are carbon!

 

[REVAN]

As an engineer, I will offer my opinion that fiberglass is the superior reinforcing fiber to use for a monofin. As for what leads to the best performance; that would have to be the design of the fin, not the material from which it is made. Good and bad monofins can be made from either material.

The main reasons I feel that fiberglass is a better reinforcing material for use in monofin applications are:

1. Fiberglass is not as strong as carbon fiber or aramid fiber (Kevlar). This sounds like a disadvantage, but actually the opposite is true in this case. Monofins need to flex in order to work properly, and for freediving we usually need them to flex a lot. Carbon fibers are so stiff that the blade must be made exceedingly thin in order to get the appropriate flexure in the blade. This will make the fin fragile. A fiberglass fin will provide the desired flexure with a thicker and much more durable laminate being used for the fin blade's construction.

2. Glass fibers in epoxy have excellent energy return from elastic deformation. Carbon fibers also have excellent energy return. Carbon and glass are very close in this metric, but the carbon costs many times more for the same performance. It is not worth it. Aramid fiber composite materials will have the lowest energy return (the highest damping) of the three reinforcing fibers and this is not what we want.

3. Fiberglass is literally glass silicon. Silicon is extremely inert and will not react with or absorb water. This is important for a diving fin that will be operating in a salt water environment. If the fibers of a composite material are allowed to wick water into the fiber, the salt can crystallize in there and slowly delaminate the fiber from the epoxy matrix. This will break down the material, destroying its elastic performance characteristics. If the fin gets cold and freezes with water in the fibers, forget it. It will delaminate fast. Fibers can be encapsulated in epoxy to protect them, but all you have to do is scratch the fin down to the fibers and the protection is lost. The best remedy is to use a fiber that can’t absorb water and glass is the one fiber that won’t. Aramid fibers have the highest affinity for water absorption and carbon, less so.

 

[Zmagoj]

As for what leads to the best performance; that would have to be the design of the fin, not the material from which it is made.
Carbon fibers are so stiff that the blade must be made exceedingly thin in order to get the appropriate flexure in the blade. This will make the fin fragile.

That’s the whole point. But would you try to convince the masses who were taught that higher costs give you better performance? But you have to admit, that thicker fin contains more epoxy and introduce bigger deformation of the surface layer, which converts a part of the absorbed energy into heat (a small part).

Zmago

 

[REVAN]

But you have to admit, that thicker fin contains more epoxy and introduce bigger deformation of the surface layer, which converts a part of the absorbed energy into heat (a small part).

The thing to keep in mind here is where you said "a small part". The performance difference is negligible; the difference in durability is not.

As I previously stated: "Glass fibers in epoxy have excellent energy return from elastic deformation. Carbon fibers also have excellent energy return. Carbon and glass are very close in this metric, but the carbon costs many times more for the same performance. It is not worth it."

I seriously doubt that a controlled experiment could detect a statistically measurable performance advantage to a carbon fiber blade over a glass fiber blade. In my opinion, the only reason the carbon fiber blades "work better" is due to the placebo effect.

 

[monkeyhatfork]

I remember a while back Dave Mullins had a thread in which he did a bunch of test dives with different bi-fins plastic,carbon,etc and from what I remember the end result was that the expensive carbons didnt perform any better than cheaper plastic fins

 

[lurker666]

I am of the opinion that /\ direction of the fibre results in stronger force needed to bend the fin. The reason is that fibres of both directions contribute to that stiffness. In mathematical terms (theoretically) that kind of orientation results in about 40% higher stiffness in the blade bend direction.

2*cos(45°)=1,41 -> 41% stiffer

I also started a thread in bifin section about self-made fiberglass fins, where I would like to discuss these aspects.

Regards,
Zmago

Old thread, I know, But I was looking in to building fins. (And using the search function as any good noob :wave )

Zmago, you might know by now that what you stated here is not true for fibre materials, just wanted to point that out, if other people are looking for info on building fins or other composite structures. (This is why you put UD in your later prototypes)

Here is a diagram that describes the performance of fibers in relation to their orientation. As you can see fibers oriented at 45° has ~5% of their stiffness (in tension)

Zmago, like your builds btw. Nice!